1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device implementing in-plane switching (IPS), where an electric field applied to liquid crystal is generated in a plane parallel to a substrate.
2. Discussion of the Related Art
Recent liquid crystal display devices use the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Liquid crystal molecules have a definite orientational alignment as a result of their long, thin shapes. That orientational alignment can be controlled by an applied electric field. In other words, as an applied electric field changes, so does the alignment of the liquid crystal molecules. Due to the optical anisotropy, the refraction of incident light depends on the orientational alignment of the liquid crystal molecules. Thus, by properly controlling an applied electric field a desired light image can be produced.
While various types of liquid crystal display devices are known, active matrix LCDs (AM-LCDs) having thin film transistors and pixel electrodes arranged in a matrix are probably the most common. This is because such AM-LCDs can produce high quality images at reasonable cost.
Recently, light and thin liquid crystal display (LCD) devices with low power consumption are used in office automation equipment and video units and the like. Driving methods for such LCDs typically include a twisted nematic (TN) mode and a super twisted nematic (STN) mode. Although TN-LCDs and STN-LCDs have been put to practical use, they have a drawback in that they have a very narrow viewing angle. In order to solve the problem of narrow viewing angle, in-plane switching liquid crystal display (IPS-LCD) devices have been proposed. The IPS-LCD devices typically include a lower substrate (often referred to as an array substrate) where a pixel electrode and a common electrode are disposed, an upper substrate having no electrode, and a liquid crystal interposed between the upper and lower substrates.
A detailed explanation for operation modes of a typical IPS-LCD device will be provided referring to FIGS. 1 to 5.
FIG. 1 is a cross-sectional view illustrating a typical IPS-LCD device. As shown in FIG. 1 lower and upper substrates 30 and 32 are spaced apart from each other, and a liquid crystal 10 is interposed therebetween. The lower and upper substrates are called array and color filter substrates, respectively. Pixel and common electrodes 34 and 36 are disposed on the lower substrate 30. The pixel and common electrodes 34 and 36 are parallel with and spaced apart from each other. Although not depicted in FIG. 1, a color filter is usually disposed on a surface of the upper substrate 32 and opposes the lower substrate 30. The pixel and common electrodes 34 and 36 apply an electric field 35 to the liquid crystal. The liquid crystal has a negative dielectric anisotropy, and thus it is aligned parallel with the electric field 35.
FIGS. 2 to 5 conceptually illustrate operation modes of a conventional IPS-LCD device. Referring to FIGS. 2 and 3, when there is no electric field between the pixel and the common electrodes 34 and 36, i.e., OFF state, the long axes of the liquid crystal molecules maintain an angle from a line perpendicular to the parallel pixel and common electrodes 34 and 36. Herein, the angle is 45 degrees, for example.
On the contrary, when the pixel and common electrodes 34 and 36 receive voltages, i.e., ON state, there is the in-plane electric field 35 parallel to the surface of the lower substrate 30 between the pixel and common electrodes 34 and 36, as shown in FIGS. 4 and 5. The in-plane electric field 35 is parallel to the surface of the lower substrate 30 because the pixel and common electrodes 34 and 36 are formed on the lower substrate 30. Accordingly, the liquid crystal molecules are twisted such that the long axes thereof are aligned with the direction of the electric field. Thus, the liquid crystal molecules are aligned such that the long axes thereof are parallel with the line perpendicular to the pixel and common electrodes 34 and 36.
By operating in the modes described above and with additional elements such as polarizers and alignment layers, the IPS-LCD device displays images. The IPS-LCD device has wide viewing angles and low color dispersion because the pixel and common electrodes are placed together on the lower substrate. These wide viewing angles of the IPS-LCD device are about 70 degrees in up-and-down and right-and-left sides. Moreover, the fabricating processes of this IPS-LCD device are simpler than other various LCD devices.
However, because the pixel and common electrodes are disposed on the same substrate (i.e., the lower substrate), the transmittance and aperture ratio are low. Further, there are some other disadvantages in the conventional IPS-LCD device. For example, the response time of the conventional IPS-LCDs is relatively slow and should be improved. In addition, the cell gap of the conventional IPS-LCDs provides very small margins for misalignment and should be improved to be more uniform.
For the sake of discussing the above-mentioned IPS-LCD device in detail, with reference to FIG. 6, the basic structure of the IPS-LCD device will be described in detail.
FIG. 6 is a plan view illustrating an array substrate of the conventional IPS-LCD device. As shown, gate lines 50 and 51 and common line 54 are transversely arranged parallel with each other. Data lines 60 and 61 are arranged perpendicular to the gate and common lines 50 (and 51) and 54. Gate electrode 52 and source electrode 62 are positioned near a cross point of the gate and data lines 50 and 60, and communicate with the gate line 50 and the data line 60, respectively. Herein, the source electrode 62 overlaps a portion of the gate electrode 52.
A plurality of common electrodes 54a are positioned spaced apart from each other and perpendicular to the common line 54. The common electrodes 54a communicate with the common line 54. Some common electrodes 54a also communicate with a common electrode connector 54b that is parallel to the gate line 50 and positioned at first ends of the common electrodes 54a. A first pixel connecting line 66 communicates with a drain electrode 64, and a plurality of pixel electrodes 66a are positioned perpendicular to the first pixel connecting line 66. First ends of the pixel electrodes 66a communicate with the first pixel connecting line 66, and the second ends of pixel electrodes 66a communicate with a second pixel connecting line 68 that is positioned over the common line 54. Accordingly, the common electrodes 54a and the pixel electrodes 66a are parallel with and spaced apart from each other in an alternating pattern.
FIGS. 7A to 7D are cross-sectional views taken along the line VII-VII of FIG. 6 and illustrate steps of manufacturing processes of the array substrate for use in the conventional IPS-LCD device.
Referring to FIG. 7A, a first metal layer is deposited on a substrate 1 and then patterned to form the gate lines 50 and 51 (see FIG. 6), the gate electrode 52, the common line 54 (see FIG. 6) and the plurality of common electrodes 54a. Further, the common electrode connector 54b is also formed with the common electrodes 54a. Aluminum (Al), Chrome (Cr), Molybdenum (Mo) or Tungsten (W) is mainly used for the first metal layer.
Now, referring to FIG. 7B, a gate insulation layer 70 is deposited on the substrate 1 to cover the patterned first metal layer. Next, an active layer 72 is formed on the gate insulation layer 70, especially over the gate electrode 52. The gate insulation layer 70 is usually made of Silicon Nitride (SiNx) or Silicon Oxide (SiO2), while the active layer comprises a pure amorphous silicon (a-Si:H) and an impurity-doped amorphous silicon (n+a-Si:H).
Referring to FIG. 7C, a second metal layer is deposited on the gate insulation layer 70 and active layer 72, and then patterned so as to form the source and drain electrodes 62 and 64, the data line 60 (and 61 of FIG. 6) and the plurality of pixel electrodes 66a. Further, the first and second pixel connecting lines 66 and 68 (see FIG. 6) are formed with the pixel electrodes 66a. Aluminum (Al), Chrome (Cr), Molybdenum (Mo) or Tungsten (W) is mainly used for the second metal layer, like the first metal layer.
As shown in FIG. 7C, the source and drain electrodes 62 and 64 are formed on the active layer 72 and particularly over portions of the gate electrode 52 such they respectively overlap opposite ends of the gate electrode 52. The pixel electrodes 66a are formed on the gate insulation layer 70 and spaced apart from each other. Also, each pixel electrode 66a is parallel with and spaced apart from the adjacent common electrodes 54a. 
Referring to FIG. 7D, a passivation layer 74 is formed on the surfaces of the above-mentioned intermediates. The passivation layer 74 has a high humidity resistance and a high durability in order to protect the active layer 72 from being affected by moisture, foreign substances, etc.
As described above, the IPS-LCD device includes the common and pixel electrodes 54a and 66a in the array substrate such that the wide viewing angle can be achieved. However, the aperture ratio is poor because the common and pixel electrodes 54a and 66a are formed of the opaque metal like the source and drain electrodes 62 and 64.
Further, referring back to FIG. 6, an area “A” between the data line 60 and the adjacent common electrode 54a, an area “B” between the data line 61 and the adjacent common electrode 54a, and an area “C” between the common line 54 and the adjacent gate line 51 are non-display areas. Namely, these areas “A”, “B” and “C” do not affect image display because the liquid crystal in these areas does not properly function and is not operated when the voltage is supplied to the plurality of pixel electrodes 66a. Therefore, a black matrix covers these areas “A”, “B” and “C” in a later manufacturing step. As a result, the black matrix shields these areas “A”, “B” and “C” from light generated from the backlight device (not show), thereby diminishing the brightness of the IPS-LCD device. Furthermore, in order to prevent the short circuit between the gate line 51 and the common line 54 when forming the gate and common lines 51 and 54 in the same plane, the area “C” is enlarged, thereby resulting in lowering the aperture ratio.
Still referring to FIG. 6, the common electrodes 54a adjacent to the data lines 60 and 61 are affected by the cross talk when the data lines 60 and 61 receive the signal voltages. Thus, the common electrodes 54a need to be far away from the data lines 60 and 61, and the areas “A” and “B” are enlarged to decrease this cross talk. Therefore, the aperture ratio is accordingly decreased.
As widely known, the aperture ratio is closely related to the brightness. Further, the stronger the brightness is, the more powerful the backlight device is. From these reasons, the above-mentioned IPS-LCD device has high power consumption.